The Spec And The Implementation

The DOM Traversal and Range Specification just defines the contstants and accept_node() for a node filter. The XML::NodeFilter implementation also adds the what_to_show() function to the class definition, since I think that it is a filters job to decide which node-types should be shown and which not.

Also XML::NodeFilter adds two constants which are not part of the specification. The first one is FILTER_DECLINED. It tells the traversal logic, that it should apply another filter in order to decide if the node should be visible or not. While the spec only defines the traversal logic to have either one or no filter applied, it showed that it leads to cleaner code if more filter could be used in conjunktion. If a traversal logic finds a single filter that returns FILTER_DECLINED, it should be handled as a synonym of FILTER_ACCEPT. While FILTER_ACCEPT is finite and would cause all other not to be executed, FILTER_DECLINED gives one more flexibility.

The second extension of the specification is the SHOW_NONE symbol. It was added for operational completeness, so one can explicitly switch the node type filter off (means all node types are rejected). This will cause the two calls of what_to_show have a different result:

  $filter->what_to_show( undef );     # will set SHOW_ALL
  $filter->what_to_show( SHOW_NONE ); # will not set SHOW_ALL

Infact SHOW_NONE is a NULL flag, that means it can be added to any list of flags without altering it.

  $filter->what_to_show( SHOW_ELEMENT | SHOW_TEXT | SHOW_NONE );

is therefore identical to

  $filter->what_to_show( SHOW_ELEMENT | SHOW_TEXT );

SHOW_NONE is especially useful to avoid numerically or even more ugly uninitialized values while building such flag lists dynamically.

How To write a Node Filter with XML::NodeFilter?

Actually writing a node filter becomes very simple with XML::NodeFilter: Simply inherit your specialized node filter from XML::NodeFilter and implement your implement the function accept_node(). This name is more perlish than the name given by the specification. If your implementation needs to stay very close to the specification, you can alternatively implement acceptNode(). Implementing both functions makes no sense, since accept_node() should be preferred by the traversal logic. Because of this acceptNode() will only be called if no accept_node() implementation was given.

Example:

   package My::NodeFilter;
   use XML::NodeFilter qw(:results);
   use vars qw(@ISA);
   @ISA = qw(XML::NodeFilter);
   use XML::LibXML::Common;
   sub accept_node {
       my $filter = shift;
       my $node   = shift;
       unless ( $node->getNodeType == ELEMENT_NODE
                and defined $node->getNamespaceURI ) {
          # ignore node without a defined namespace
          return FILTER_REJECT;
       }
       return FILTER_DECLINED;
   }
   1;

This example shows a simple nodefilter that will reject any element without a namespace defined. Note that FILTER_DECLINED is returned if the node was not rejected. This allows a traversal logic to apply another filter on the nodes with a namespace defined. If your application needs to use different filters on the namespaced elements depending on the state where you want to traverse your DOM but you need always to ignore elements without a namespace such a filter will enshure that you need not to add redundant code to your filter or even to choose a base class.

How To make use of a XML::NodeFilter Filter?

If you need to write some traversal code yourself, you should call the node filters accept_node() function to test if the logic should return the current node. A node is not returned if any filter retunrs FILTER_SKIP or FILTER_REJECT. In this case you need to reinvoke your traversal code.

The following code snippet shows how you can make use of XML::NodeFilter in your traversal logic:

  use XML::NodeFilter qw(:results);
  #...
  sub traversal_logic {
      my $refnode = shift;
      my @filters = @_;
      my $node = undef;
      TRAVERSE: while (1) {
            my $state = FILTER_DECLINED;
            # your traversal logic
            # ...
            last TRAVERSE unless defined $node;
            FILTER: foreach my $filter ( @filters ) {
                $state = $filter->accept_node($node);
                last TRAVERSE if     $state == FILTER_ACCEPT;
                last FILTER   unless $state == FILTER_DECLINED;
                        last TRAVERSE if $state == FILTER_DECLINED;
      }
      return $node;
  }

As you see the traversal code will call only accept_node() on each filter. Still this will work fine with filters, that have acceptNode() implemented: XML::NodeFilter calls acceptNode() if the original accept_node() function is called. This ashures that filters that use function names conform to the specification will work as well.

Note that XML::NodeFilter uses as default return value of accept_node() FILTER_ACCEPT rather than FILTER_DECLINED. This is done so you can write 100% specification conform traversal and filter logic.

Functions

new()

As the constructor of this class it accepts some parameters in form of a hash. This parameter hash will be blessed into a hash reference. The only parameter used by the class itself is -show. This parameter may hold a bitmask of node filter flags as described below or a hash reference containing the same information.

If -show is omitted SHOW_ALL is assumed as default.

what_to_show()

This function is added to the filter class, rather than assuming it is available directly from within a traversal logic. what_to_show() takes either a bitmask or a hash that holds the information what nodes should be filtered.

If what_to_show() is called without any parameter, it simply returns the bitmaks in scalar context; if called in array context it returns a hash containing the corresponding information: If a bit is set in the bitmask the corresponding key has the value 1; otherwise it has the value 0.

accept_node()

This function is used to tell a calling traversal function if a given node should be returned to the caller or should be skipped. It has four possible return values:

FILTER_DECLINED to indicate that the filter itself would accept if no other (less significant) filters rejects or skips it. NOTE FILTER_DECLINED is not defined by the spec.

FILTER_ACCEPT to indicate that a node is accepted regardless what other filters may indicate.

FILTER_SKIP to indicate a node is skipped, but its descendants should be still available.

FILTER_REJECT to indicate a node and all its descendants should be skipped by the traversal logic.

By default accept_node() returns FILTER_ACCEPT.

acceptNode()

Alternative function for accept_node(). This is only available for spec conformance. Any traversal logic should request accept_node(). Node filter implementations may choose either to implement accept_node() or acceptNode(). Implementing both makes no sense at all!

Constants

• FILTER_DECLINED (0)

  Additional symbol to allow stacked node filters.

• FILTER_ACCEPT (1)

  Defined by the specification

• FILTER_SKIP (2)

  Defined by the specification

• FILTER_REJECT (3)

  Defined by the specification

• SHOW_ALL

  Defined by the specification

• SHOW_ELEMENT

  Defined by the specification

• SHOW_ATTRIBUTE

  Defined by the specification

• SHOW_TEXT

  Defined by the specification

• SHOW_CDATA_SECTION

  Defined by the specification

• SHOW_ENTITY_REFERENCE

  Defined by the specification

• SHOW_ENTITY

  Defined by the specification

• SHOW_PROCESSING_INSTRUCTION

  Defined by the specification

• SHOW_COMMENT

  Defined by the specification

• SHOW_DOCUMENT

  Defined by the specification

• SHOW_DOCUMENT_TYPE

  Defined by the specification

• SHOW_DOCUMENT_FRAGMENT

  Defined by the specification

• SHOW_NOTATION

  Defined by the specification

• SHOW_NONE

  Additional symbol to indicate a NULL filter. This is for operational completeness.

• @FLAGNAMES

  Contains the names of the FLAGS used by what_to_show(). The combining symbols SHOW_ALL and SHOW_NONE are not included by this list.

• %FLAGMAPPING

  This hash mapps flagnames (as listed by @FLAGNAMES) to their predefined values. The combining symbols SHOW_ALL and SHOW_NONE are not included by this list.

  NOTE: @FLAGNAMES and %FALGMAPPING are not exported. To make use of them you have to use the fully quallified namespace as follows

    # gives the value of the SHOW_ELEMENT.
    my $flag = $XML::NodeFilter::FLAGMAPPING{SHOW_ELEMENT};